1. Field of the Invention
This invention relates to a cathode for an electron tube such as a cathode ray tube and particularly to an improvement in electron emission characteristics of the cathode.
2. Description of the Prior Art
In the prior art, a most commonly used cathode for an electron tube such as a picture tube is the so-called oxide cathode in which an alkaline earth metal oxide layer containing Ba is formed on a base of Ni as a major element containing a small amount of a reducing agent such as Si or Mg.
An electron-emissive layer of such an oxide cathode is an oxide layer obtained by conversion through thermal decomposition of an alkaline earth metal carbonate. Thus, the oxide is caused to react with the reducing agent so that free metal atoms are generated to serve as donors for emission of electrons.
In the above described process, a carbonate, which is chemically stable, is used as the starting material instead of using BaO from the beginning. This is because Ba is very active and BaO is liable to react with moisture in the air to produce Ba(OH).sub.2, from which it would be difficult to obtain free Ba in an electron tube.
The carbonate includes a single component such as BaCO.sub.3 or a multicomponent such as (Ba,Sr,Ca)CO.sub.3. Since the fundamental process of forming donors through activation is common to both cases of the single component and the multicomponent, an example of using a single component carbonate will be described in detail hereinafter for easier understanding.
FIG. 1 is a schematic sectional view illustrating an example of a conventional oxide cathode. A cathode cylinder includes a cap formed of a base metal 1, and a cylinder 2, and a heater 3 is provided inside the cathode cylinder to heat the cathode. An electron-emissive layer 55 of BaO is formed on a surface of the base 1.
The electron emissive layer 55 is formed by a process as described below. A resin solution obtained by dissolution of nitrocellulose or the like into an organic solvent is mixed with BaCO.sub.3 and then the mixture is put on the base metal 1 by such a method as spraying, electrodeposition or application.
The cathode thus formed is incorporated in an electron tube and then it is heated to about 1000.degree. C. by the heater 3 in an evacuation process for evacuating the electron tube. Thus, BaCO.sub.3 is thermally decomposed and converted to BaO as indicated by the following formula I. EQU BaCO.sub.3 .fwdarw.BaO+CO.sub.2 (I)
Gas of CO.sub.2 produced by the reaction as well as other gases produced by thermal decomposition of nitrocellulose is removed outside the electron tube.
However, the above described process involves disadvantages that the reducing agent of Si or Mg having an important role in reduction is unavoidably oxidized in an oxidizing atmosphere of CO.sub.2 or the like in the tube at the time of the reaction represented by the formula I and that the element Ni of the surface of the base metal 1 is also oxidized on that occasion.
FIG. 2 is an enlarged fragmentary sectional view illustrating an interface region between the base 1 and the electron-emissive layer 55 for explaining the interface region in detail. In general, BaO constituting the electron-emissive layer 55 is in the form of aggregates 9 of several .mu.m to several tens of .mu.m in size formed by gathering of small prismatic crystals 8. Desirable gaps 10 are provided between the respective adjacent aggregates 9 of the electron-emissive layer 55, which makes the layer 55 porous. The substance BaO reacts with the reducing agent of Si or Mg in the interface region 11 where the layer 55 and the base 1 are in contact, so that free Ba is produced. The reducing agent diffuses along grain boundaries 7 of Ni crystal grains 6 of the base 1 and reduction reactions II or III as expressed below occur near the interface region 11. EQU 2BaO+Si.fwdarw.2Ba+SiO.sub.2 (II) EQU BaO+Mg.fwdarw.Ba+MgO (III)
The free Ba thus obtained serves as a donor for electron emission. At the same time, reaction represented by the following formula IV also occurs. EQU SiO.sub.2 +2BaO.fwdarw.Ba.sub.2 SiO.sub.4 (IV)
Free Ba serving as a donor as described above is generated in the interface region between the electron-emissive layer 55 and the base 1 and it moves through the gaps 10 in the electron-emissive layer 55 and comes out on the upper surface of the layer so that electrons are emitted. However, since the donors are evaporated or are consumed as a result of reaction with residual gas such as Co, Co.sub.2, O.sub.2 or H.sub.2 O, it is necessary to constantly supply donors by making the reactions as expressed by the formulas II or III. Such a cathode is generally used at a high temperature of about 800.degree. C. so that a good balance is maintained between the supply and the consumption of the donors.
However, reaction products 12 such as SiO.sub.2 or Ba.sub.2 SiO.sub.4 represented in the formula II or IV are generated in the interface region 11 during operation of the cathode: Consequently, the reaction products 12 are accumulated in the interface region 11 and the grain boundaries 7 increasingly during the operation of the cathode to form a barrier (generally called an interface layer) against Si or the like moving in the grain boundaries 7. As a result, the reaction becomes gradually slow, which makes it difficult to generate Ba as the donor. In addition, this interface layer has a high resistance value, causing obstruction to electron emission current.
In order to solve the above described difficulties, prior art documents such as Japanese Patent Laying-Open No. 269828/1986 or Japanese Patent Laying-Open No. 271732/1986 disclose the below described techniques making use of the formation of an electron-emissive layer including dispersed powder of Sc.sub.2 O.sub.3, in which:
(1) a composite oxide (for example, Ba.sub.3 Sc.sub.4 O.sub.9) produced as a result of reaction between Sc.sub.2 O.sub.3 and an alkaline earth metal oxide is thermally decomposed during operation of a cathode so that free Ba as the donor is generated and supplied;
(2) free metal scandium (Sc) is used to enhance conductivity of the electron-emissive layer; and
(3) reaction products such as Ba.sub.2 SiO.sub.4 in the interface region are decomposed by Sc.
Thus, according to the above described prior art, a cathode for an electron tube can be operated with a high-current density by virtue of the electron-emissive layer including dispersed powder of Sc.sub.2 O.sub.3 ; however, there are involved disadvantages such as variations in electron emission characteristics of the products manufactured. In addition, it sometimes happens that the powder of Sc.sub.2 O.sub.3 is not sufficiently uniformly dispersed in the electron-emissive layer, making it difficult to obtain a sufficient amount of electron emission current.